Conventional soil solarization is a technique that involves the use of solar power for controlling disease agents in the soil and decontaminating soil using sunlight; e.g., mulching the soil and covering it with a transparent polyethylene tarp to trap solar energy and therefore heat the soil.
Control of plant adversaries such as nematodes, disease causing bacteria, fungi, weeds, and soil-borne pests is beneficial for healthy plants and improved yields in agriculture. Pre-plant treatment of soil by fumigants is very effective for this purpose and has been widely used in the US and throughout the world, along with conventional agricultural practices. The additional cost of fumigation originating from labor, pesticides and materials, and machine use was compensated for by increased crop output and quality.
Soil fumigation was typically accomplished with the use of methyl bromide (MB) as a fumigant. Methyl bromide, however, is an extremely toxic chemical; its exposure experimentally linked to higher rates of cancer. Documented effects of MB exposure range from skin and eye irritation to death with most fatalities and injuries occurring when methyl bromide was used as a fumigant. In addition to its acute and chronic health risks, methyl bromide is a highly active green-house gas and has been classified as a Class I stratospheric ozone-depleting chemical. Therefore, the use of MB and related ozone-depleting compounds has been banned by the 1987 Montreal and 1997 Kyoto Protocols.
Notwithstanding the phase-out regulations imposed by international treaties, MB fumigation has been allowed through ‘critical use exemptions,’ which allow continued use of MB when there is not an adequate alternative. These exemptions, along with the large outgas sing of MB-treated products and containers shipped internationally, are a source of MB related health problems to people who are far from the site of original fumigant application of MB.
Other fumigants like metam sodium and chloropicrin, which can be considered as alternatives to MB, are considered not as effective and are also controlled by regulations that prohibit the use of fumigants in certain geographical areas that are close to schools and houses. Although fumigation is a costly process and may necessitate weeks long airing time, not having an economically viable method of a soil pretreatment agent such as MB has had a negative effect on growers' net income. For instance, in 2008 California strawberry growers using alternatives incurred a 14% decline in gross revenue in comparison to those using MB. As the number of critical use exemptions for MB fumigation decrease in the coming years due to MB phase-out regulations, an additional adverse impact on the $1.8 billion strawberry industry, for example, can be expected. Industries for other high-value crops that had relied on MB fumigation will also experience losses. Therefore, there is an immediate and strong need for a technically and economically viable method (and related apparatus) for soil disinfection/sterilization/de-weeding that is sustainable, organic, and is not toxic to people or the environment.
Conventional soil solarization is performed by covering soil with plastic sheets and sealing them around the edges. Natural sunlight heats the soil and the plastic traps the heat under the plastic. This process can take several weeks (3-8 weeks are typical) for satisfactory disinfestation/pasteurization results. The temperature inside the tarp is highly dependent on the weather conditions (outside temperature, day/night time, cloudiness), soil characteristics (composition, type, humidity), and plastic thickness. Temperature is highest at or near the surface of the soil, with reported typical (for a site in California, USA) maximum/minimum temperatures of 50/37° C. at 10 cm and 43/38° C. at 20 cm. Soil temperatures as high as 70° C. have been reported for bagged, isolated, or thin (<12″) piles of soil under certain plastic covering conditions.
Soil solarization is a tremendous improvement over using chemical fumigation in terms of environmental impact; however, it still requires a large amount of plastic, which needs to be disposed after use as small holes and damage over three to eight weeks of solar exposure treatment makes reuse impractical. Often times the plastic is buried or burned, leading to a significant environmental impact. Biodegradable plastics are not used due to their higher opacity to sunlight, less durability to installation, and higher costs.
In addition to the environmental impact of the plastics used, there are several drawbacks of conventional soil solarization that make it less attractive than chemical fumigation to farmers:
a) Treatment time: In general, three to eight weeks of soil heating during the warmest time of the year is sufficient to control most soil pests. In some cases, such as in cooler, windier, or cloudier locations, or if there are pests that are harder to control, it may be necessary to leave the plastic in place for three to eight weeks. This is a significant penalty to the farmer in terms of time lost for crop growth. This lost time period may limit the number of crop cycles that can be harvested in one year, reducing farm yield.b) Performance/(Cost/Acre): Even though the cost of conventional soil-solarization is lower than that of chemical fumigants, the performance achieved is not sufficient to substantiate the lower cost.c) Applicability: Soil solarization is practical only in hot climates with sufficient sun exposure; e.g., California, Arizona, Florida. A farmer in New York or Ohio cannot regularly benefit from soil solarization as the sunlight is not sufficient in the early spring and late fall seasons of crop planting. In addition, solarization efficiency can be reduced by cloud cover, cool air temperatures, and precipitation events during the treatment period.
Solarization as described hereinabove is a passive process whose incident power is limited by that of the natural sun-light; i.e., approximately 1 kW/m2 at its maximum. This can be increased by concentrating the solar power to increase the solar power density by X suns. The literature shows that the time required to kill a certain fraction of a population of a certain species of a weed is a very steep function of temperature, with only a 4° C. decrease in temperature having an impact on the success of de-weeding. Many thermal processes are nonlinear functions of temperature; for example, DNA unravels from a double-strand state to single-strand state at 90° C. in polymerase chain reaction. Many proteins denature at high temperatures above a certain threshold. Hence treating soils to higher temperatures can accelerate soil sterilization. Achieving the higher temperatures can be achieved by concentrating sunlight to increase the power to the exposed surface.
Early use of concentrated sun-light for solarization was shown by Johnson et al. in 1989. As shown in FIG. 1, a Fresnel lens was used to concentrate sunlight by a 25× factor over a rectangular region of 1 cm×150 cm. The authors demonstrated that within one second of exposure, the surface temperature could rise to 290° C. This heat treatment over an exposure time of 1-10 seconds was sufficient to kill kochia, redroot pigweed with close to 100% efficacy. The high temperature at the surface causes a sufficient thermal gradient in the soil to result. Surprisingly, Johnson et al. achieved better results in dry soil, compared to wet soil, even though dry soil has more reflected light from the soil. The experiments performed on the seed stage were even more effective. The authors showed that a 20 second exposure to seed on the soil surface was 100% lethal to green foxtail, kochia, common lambsquarters, common purslane, and wild buckwheat.
This work by Johnson et al. demonstrated that concentrated solarization can have significant efficacy for weed control. However, they further concluded that a one second exposure, which equated to 36 meters/hour, was too slow for practical field application. Another recognized obstacle was the narrow depth of focus, which lead to poor control as plant canopy expanded at the later growth stages. A larger linear lens producing higher temperatures, as additional lens system to increase the depth of focus, should be considered for future considerations, according to the authors.
Phitthayarachasak et al. demonstrated another example of concentrated solarization by using an asymmetric compound parabolic concentrator. The setup shown in FIG. 2 performs soil irrigation with hot water (60-70° C.) in addition to focusing sun light on the soil surface. The hot water is heated through long copper tubes that are in contact with the collector surfaces exposed to the sun. Water is dropped at both the surface and at 30 cm depth. Recorded surface temperatures exceeded 50° C. They were able to decrease typical solarization times of four to six weeks down to four hours, while reducing the bacterial population density of Ralstonia solanacearum, the causative agent of wilt in crops leaves, more than five orders of magnitude at the surface. In addition, they demonstrated that temperature of the soil at different depths could be more uniformly increased if heated water was injected into soil at different depths. On a large scale, the generation of heat using electricity or gas in a combustion based boiler would be costly and physically unwieldy.
Referring to FIG. 3, a US patent application titled “Method and apparatus for controlling weeds with solar energy” by Guice et al. reveals air, ground and sea vehicles to expose weeds to (concentrated) solar energy. It discloses methods to control the intensity of light using robotic and autonomous systems, which are equipped with GPS or satellite based navigation and control. The importance of having instruments and vehicles solar powered (through converters such as PV cells or Stirling engines) to minimize gas consumption is disclosed in the publication.
In the foregoing discussion, various approaches attempting to meet and solve the aforementioned challenges and problems were presented. The inventors have recognized the benefits and advantages of providing solutions to these challenges and problems that are efficacious, scalable for practical agribusiness applications, and economically feasible; solutions not provided by past attempts.